fix(test): make pytest config parse on current pytest

[guyua9]

Summary

• move pytest settings to the supported tool.pytest.ini_options table
• make addopts a TOML list so current pytest can parse it
• add pythonpath = ["src"] and a collect-only regression test from the repo root

Why

With current pytest, the repository fails before test collection because addopts is stored as a string in pyproject.toml:

TypeError: ... config option 'addopts' expects a list for type 'args', got str: '-v'

That blocks even targeted unit tests. This keeps the same -v behavior while using the TOML shape current pytest expects.

Tests

• uv run pytest tests/unit/test_pytest_config.py tests/unit/test_state_files.py -q
• uv run --extra dev ruff check tests/unit/test_pytest_config.py
• uv run --extra dev ruff format --check tests/unit/test_pytest_config.py
• git diff --check